Those red-eye flights and all nighters may be leaving their mark in your hair. Researchers have found that hair follicles contain a signature of the 24-hour circadian clock that sets our sleeping habits. The method could one day help track patients with sleep disorders and help evaluate health problems in late-night shift workers.

At one point, researchers thought that the circadian clock was located solely in the brain. But after scientists discovered human circadian clock genes in the late 1990s, they found that the genes were expressed in tissues throughout the body. In experiments with mice, researchers have linked these genes to weight gain[2] and even to the "lost in time" feeling of marijuana use[3], but they've had a harder time studying them in humans. That's because analyzing these genes relies on invasive methods, such as drawing a person's blood several times a day or excising a small chunk of skin.

Makoto Akashi of Yamaguchi University in Japan and his colleagues sought an easier way to check clock gene activity. They turned to hairs plucked from scalps or beards, which contain cell-rich follicles. When they extracted RNA from these cells, they found that circadian gene activities peaked when volunteers were awake and alert, and it peaked earliest in the volunteer who woke up earliest in the morning.

Next, Akashi and colleagues disrupted the sleep-wake cycle of healthy people: They asked volunteers to sleep in later and later over a 3-week period and to shine a bright light on themselves to mimic sunlight for half an hour after they awoke. At the end of 3 weeks, when the volunteers were waking up about 4 hours later than they used to, the activity of their hair follicle circadian genes had shifted too—but only by about two and a half 2½ hours, the team reports[4] online today in the Proceedings of the National Academy of Sciences.

Akashi's group saw a similar lag in shift workers. The team examined the hair follicles of volunteers who worked the 6 a.m. to 3 p.m. shift one week, the 3 p.m. to midnight shift the next week, and back to the morning shift on their third week. Clock gene activity lagged 5 hours behind the workers' lifestyles, the researchers found, which indicates that 3 weeks was not long enough for the body clock to adapt to the new schedules.

Tracking clock genes in hair follicles could help researchers better monitor patients with sleep disorders and other circadian rhythm dysfunction, says molecular biologist Ueli Schibler of the University of Geneva in Switzerland. Because these genes control everything from organ function to eating cycles, he says, the lag the team observed in shift workers could help explain some of the serious health problems they develop.